A typical mobile network such as a standard Global System for Mobile (GSM) network comprises several base stations, or base transceiver stations (BTS), which communicate in with mobile units, each mobile unit transmitting on a single frequency or carrier and receiving on a separate single frequency or carrier. Clusters of base transceiver stations are controlled by a base station controller (BSC). The BSC assigns frequencies and/or time slots (as well as codes if appropriate) to each connection, minimizing co-channel and adjacent channel interference. The BSC and/or the local BTS monitor the strength of the signal from each mobile unit in contact with a BTS to determine which BTS should be assigned to that mobile. Mobile units can also provide feedback on received signal strength, carrier to noise ratio observed by the mobile unit. This information on the propagation environment is used to balance the communications load and assign each mobile unit to a BTS. The assignment generally includes information to enable the mobile unit to share access to the BTS in the form of a time slot, a frequency or frequencies, a code, or a combination thereof, generally depending on the communications protocol being implemented.
One aspect of mobile communication relates to handover in which the connection for a mobile unit is switched from one BTS to another BTS. Handover may take place between neighboring BTS or from one sector covered by a BTS to another sector of the same BTS. All discussions of handover can be for either case. Hard handover is one type of handover commonly used in systems where multiple users share access by time division multiple access (TDMA) or frequency division multiple access (FDMA). When hard handover occurs, the mobile unit severs all contact with the existing base station and then establishes a session with the new base station. In a GSM network, the mobile unit switches frequencies for both transmit and receive every time slot and also when handoff occurs.
In mobile assisted handoff (MAHO), the mobile unit, which only receives voice or data information during a sub-set of the time slots broadcast by the BTS (one in eight for a GSM voice connection), can sample signals from neighboring base stations during the unused time slots to provide additional information. These signals are power controlled for other users and can fluctuate rapidly. If the mobile is receiving data via GSM Packet Radio Service (GPRS), it may receive data during most of all of the time slots. Handoff may be initiated by the BSC, BTS, or the mobile unit depending on system design. In either case, feedback from the mobile unit is useful in deciding when to initiate handoff.
Hard handover is less desirable than soft handover because the existing connection between a BTS and a mobile unit is completely severed and a completely new connection must be initiated within a constrained amount of time, typically 200 ms in a GSM network. If the system does not perform nominally during the handover period there is a good chance the connection will be dropped. Soft handover has been implemented in wireless systems using code division multiple access (CDMA) that allows neighboring base stations to share frequencies.
Unexpected loss of signal from the mobile to its BTS or from the BTS to the mobile (which can occur separately or together) often results in a dropped call. Current systems using either soft handover, or hard handover in particular, are vulnerable to dropping calls due to unscheduled loss of signal.
Soft handover allows some level of communications to be established between a mobile unit and a new base station prior to the disconnection of the mobile unit from its existing base station. Soft handover is less susceptible to dropped calls and the latency for establishing a connection to the new base station is also reduced. Thus, a method for soft handover is desirable, particularly for GSM and other TDMA and FDMA wireless standards. Having established connectivity in at least one direction with a second BTS enables the system to be much more robust against dropping calls due to sudden loss of signal.
Mobile users may be denied a connection if they attempt to initiate a call where the primary coverage (e.g. strongest signal) is from a BTS that either can not receive a signal from the mobile (note the transmit and receive links are not symmetric) or if the BTS is using all of its assigned frequencies and time slots. Similarly, a mobile user that moves into a new area that nominally has coverage may be dropped if the BTS providing the coverage does not have available capacity to transmit and/or receive. In both these cases other base stations may be in range to communicate with the mobile and a system enabling a mobile to communicate with more than one BTS would enable superior load balancing and deployment of network resources.